Hardest disision I have ever encountered please help?

yesterday my mastiff played to rough with my bichon frise they play rough alot they love to play with each other but yesterday molly the bichon frises knee cap came out of joint we had to take her to the ER and thry said its from the playing and if it gets anyworse they will have to operate we cant afford the surgery its realy expensive we are low on money because my mom is disabled and my dad died last year I am only 13 and my mom said we might have to get rid of my mastiff my dad gave my mastiff to me as a present 4 months befor he died but my bichon frise was my dads dog this is so hard I love my mastiff but I cant let my bichon frise die she is only 3years old I dont know what to do