Good Afternoon and welcome! I am Dr. Loretta, a licensed veterinarian and I am happy to answer your question.
There are tumors that will grow inside the mouth and these can be malignant or benign. They can also get an Epulis which is a benign tumor within the mouth that can be removed. This LINK explains it in more detail This LINK shows what it may look like.
The best thing your friend can do is to have a biopsy if this mass and that will tell your friend what to do...if it is benign, they can be removedand if this is an abscess, it can be treated.
Does that make sense to you?
Please let me know if you have ANY other questions. My goal is to give you 100% satisfaction and if you are not yet satisfied, please REPLY so I can clarify for you.
Sincerely, ***** *****
Thank you so much for your kind rating. It is very much appreciated! Even though you have rated, we can still communicate. If you have other questions, please Reply. I am always happy to help. I hope Spot's mass is not serious.
Have a wonderful day!