If it's a salivary gland cyst (sialocoele), it should come out. Our bluetick just had one (only got to be about golf-ball sized and we noticed it very early on---only about 3 days since it had started), and luckily we had a vet that did the surgery for $150. I did quite a bit of research on this, and there are some good web resources. Just do a google search on one of the names above. Some places have vets that are much more expensive than the ones in our area.

The salivary glands are located on either side of the face at the back of the jawbone (where the curved part of the jawbone meets the neck), and under the jaw (in the middle). The reason for these cysts is that the salivary ducts get backed up (for whatever reason...can be trauma, but usually you can't tell the reason by the time it gets looked at). The saliva backs up in the gland, since it can't escape into the mouth, forming a fairly hard lump to start with. It can grow to a very large size (I found some alarming pics during my research). The problem with these is that if they go too long untreated, they can rupture and let all those salivary digestive enzymes loose into the surrounding muscle tissue (kind of like an appendix does when it bursts). Once this happens, it's much harder to do the necessary surgery.

The surgery for Maggie was an all-day affair, then a maintenance job for about 2 weeks. She had drainage tubes in her neck (which was really gross), had to have an antibiotic pill daily, and antibiotic cream on the place he cut. The tubes came out in 1 week and the rest of the stitches at 2 weeks and she is back to her normal self with a very small scar on her neck (which will probably disappear in time).

Good news, though, is that they do not usually come and go like the lump on your friend's dog. I tend to suspect allergies any time that things come and go like that, but I am very allergic to lots of things, so I may be a bit of a allergy hypochondriac. I would try to get the dog to a vet while it has the bump; forget about appointments and just take him as a walk-in and explain about the coming and going part after the fact.