May I write a letter to support my girlfriend's application? My girlfriend and I am in the same college, but different major. She is a law school master student.

She just graduated from the master program and wanted to transfer to juris doctor in the same college.

I am wondering if it is appropriate for me to write the dean of law school a letter in support of her application.

Also, I am not sure what to write in the letter if appropriate. I would imagine saying that I would love to be able to continue studying with my girlfriend in the same college and things like that. Would you have any suggestions?

Thank you in advance!
Just say no.  No, it's not appropriate.  This is not even close.  She might as well ask her mother to write the LOR.  "She was such a good girl growing up."

It's possible it might not get discovered but if it is, I expect it would (should!) quickly tank her application before most admissions committees.  She needs to find references who can speak to her qualifications without this obvious conflict of interest.