I know you asked for home remedies, and this isn’t one of those, but I thought I would share just in case your girlfriend is looking into options for mosquito bite avoidance in the future, or in case anyone reading this is looking for information!
If you haven’t already tried this, get yourself some 100% DEET. It’s a fairly extreme solution – the stuff is extremely strong and not nice, but it works extremely well. It is a solvent, so it is corrosive to some things (e.g., plastics, leather, etc), and it may irritate your skin if you apply too much or have broken/sensitive skin, but it is so effective. Insect repellent sprays and lotions usually contain DEET in small amounts as it is the active ingredient, so if you can get yourself some of the pure stuff, it really does the job. I wouldn’t recommend using it for long periods of time, mind you!
I always find myself the target of insect bites, all year around, and this is the only stuff I bother with these days. Regular chemist insect repellents just don’t work for me at all or wear off very quickly, so I stopped wasting my money!