In this case the safest bet (you should still check with the artist about the kind of paint used) would be to utilize a waterborne sealer.
The technology has come a long way and is as durable if not more so than old fashioned sealers (poly, lacquer etc). As additional bonuses, the smell is far less offensive, it dries more quickly, and is not prone to yellowing.
As far as application goes I would suggest spraying it on if you have the facilities/tools. An HVLP sprayer is great for projects like this, it conserves material when you are working on small objects.
To avoid any damage to the artwork apply very light coats until you have a full seal, and do NOT sand until you have a solid base of coating built up. Sand light with large numbered grits (at least 200 probably, or 000+ steel wool).
Once you have a full seal you can apply thicker coats that take longer to dry.