sxe couple, I dont know if I can help or not as im not the best with describing things but I "self taught" after seeing porn and thinking ooooh interesting, can i? So i shall try help you guys with my experience.
First of all if she is feeling an overwhelming urge to pee, your halfway there! When I first started trying to make myself orgasm/squirt I would always get this feeling, however nothing happened much more than that. Just this urge to pee. It got frustrating to be honest, I tried quite a few times, different occasions and one day while trying I just got pretty frustrated and started going faster and harder then just pulling the toy out of me in annoyance....and erm....yeh I squirted. It didnt feel particularily nice or amazing or anything, I was confused as to what had happened to be honest lol I knew I hadnt just pee'd.
Since those first experiences I have come to learn to read my body a bit more and understand a bit more what the feelings mean and whats going on. For example one thing I have to say (and I dont think I am alone here) is that you can happily bang away at the G-Spot with toys and yes it feels amazing and these feelings are building up but if you are actually looking for your partner to "squirt" you need to remove the toy at the "right moment" otherwise she will still be feeling those lovely feelings for sure but having something in the way seems to (for me anyway) stop any "squirting" you may get dribbles happening but ...yeh.....fingers on the other hand being a bit smaller wont stop your lady from squirting and fingers are great for finding that spot hehe. In fact I would suggest doing it with fingers until your lady gets a sense of the feelings and when she feels those amazing feelings and when she starts to let go as with fingers you can see and feel it all happening between the two of you.
Ok now onto the pee feeling and the reason I went from "Unable" to "able" to squirt. When you are in the middle of playing and you get this over riding urge to pee, your initial natural instincts conscious or unconscious are to hold it, squeeze your muscles to stop from peeing. However, this is the part you need to get your mind over, its not actually an urge to pee its an urge to let go and squirt. Your G-Spot area is filling with fluid due to being teased and with the continued pressing your lovely lady is feeling that. Squirt comes out thru the urethra the same as pee, and you are pressing so close to the bladder its hard to tell the difference in those feelings. so yes it is a bit of a mental hurdle when you start to feel those urges not to clamp up. In fact you have to do the opposite. Also, and this is pretty important for me, I dont know about other ladies but your lady, when she reaches those intense intense points of ohhhhh myyyy gooooddd, she should almost feel an urge to push/bear down? (she might not be feeling this at all right now as the sensations and urges to pee will be stronger) rather than tense her muscles to hold it in...she needs to almost let that "bearing down" or "pushing" feeling take over and go for it. You know the muscles you use for pushing out a vibrator inside you? Well when I get close, i used to get an urge or consciously use that "push the toy out" muscles when it was feeling really intense and thats when the toy should be removed when you feel that pushing...remove the toy and hopefully you will have the intended reaction.
It takes some patience, some practice, some working through thoughts like "well this doesnt seem all its cracked up to be", frustrations etc but perseverance won for me and eventually I learned not only could I do this but it feels amazing! Now I am much more in touch with whats going on with the feelings, the urge to pee, the muscles working against you, and now that I "learned" to do it, it is now completely natural I now know I dont need to pee, I dont feel urges to push or urges to tense up...just because mentally I am completely relaxed about it it happens so much more naturally and feels so much better than it used to do back in the day of learning lol
one other small little detail to help overcome the mental block, you know what your aim is (squirting) you know its going to get wet and messy hehe but reassure your lady its part of the fun and if....IF....she did pee, whats the worst that could happen? I did once! and you can tell the difference lol, my boyfriend decided to play with me that way while I was desperate for a pee and I managed to do both, I was slightly embarrassed and he still likes to wind me up about "peeing on him" but stuff happens, we turned it into something to laugh about because really, if your doing something thats going to be pretty wet anyway and a little accident (lol) happens just roll with it and carry on 
be somewhere where you are not worried about damage or mess to bedding or furniture, like someone suggested maybe the bath or shower but of course if you are looking for that "magic moment" try it without the water running so you know when that special moment is happening. (I dont feel the liquid leaving my body i just feel the intensity inside, I only know I have when I see/feel the wet patch/spray etc so having the water running might mean you miss it if you are new to this)
I dont know, ive rambled on for a few paragraphs, I hope I might have been some help, Just my personal feelings and experiences with this and would really love to help another lady enjoy this pleasure (thru this message, I am not turning up at your door with a g spot vibe hahaha) Let us know if you get anywhere!!
