Hi I Have A Dog That Has Been Moaning And

hi i have a dog that has been moaning and in pain when she lays down. she is an american bulldog and has a very high pain threshold. i took her to the vet and he did xrays statin gthat her heart is very enlarged and she has tumors on her liver. i was wondering if i should let her keep going until she finally passes on her own take into acct that she is still eating and doing great while standing up its just when she lays down that it hurts. we have her on pheno or would it be kinder not to put her through this and have her put to sleep. we love her so much she is 13 years old