How do I get LOR from a project mentor who's still pursuing PhD since she doesn't have an official email or access to the University letterhead? I plan to apply for Masters. I am currently working as a research intern at a reputed University in my currently. I was assigned a Ph.D. mentor on joining the project by the Professor. Ph.D. mentor guided me on a daily basis and has a better understanding of my work and abilities than the Professor. The professor has little knowledge about my capabilities and I'm afraid he'll give me a very weak LOR if at all he agrees to give. Is it advisable to get a LOR from my Ph.D. mentor considering she does not have an official University email address or the University letterhead?
Although it looks more professional to have a faculty email address on a letter of recommendation, it is not a must.

The person that you are asking might have left the university, and also they might not yet have started in another university. This would not prevent you from having the letter of recommendation.