I did my open water referal dives with Eddie Alamilla from Amigos Del Mar last October. My first dive was at Hol Chan, shallow and a very easy dive depending on the current. My other three dives were at Pillar Corals and San Pedro Canyons. We only spent 5 or 10 minutes each dive working on skills. The other 30 minutes we just swam around. There should be no problem joining your girlfriend on her dives.
ADM is a great dive operation but you will probably have 6-8 divers on each boat. I did a night dive and there were 10 divers.
When I went back in December I dove with Protech, just wanted something different. Protech was a great small operation. There were two girls on my boat that were doing their open water referal and they seemed to like Protech. The night dive I did with Protech was great. It was just the divemaster and my self. On all my other dives there were only two or three divers.
I am sure that all of the dive shops in SP are good but do not dive with Aqua Dives Belize. I have spoken with dive shop owners and other dives and nobody has anything good to say about Aqua Dives.
No matter who you dive with, just go and have fun.