I'm delving back into my memory banks for this, so I may not have tha answer 100%....but...varnish is used on Fibre-Based prints. A while back, FB paper only came with one finish, which was a semi-gloss. To "gloss" the paper up, one varnished it when mounting, I think.
I use two Krylon products for protecting prints from inkjet printers.
The glossy version is Krylon's Crystal Clear Acrylic Coating, #1303.
The matte version is Krylon Matte Finish, #1311.
Both work very well. I strongly suggest coating both sides of the paper to equalize moisture penetration rates and minimize (usually eliminate) any curling that might take place with a single-sided application.
Hmmm...I've never heard of varnishing any silver gelatin process. However, I can positively tell you that practitioners of the (very old) wet plate colloidon process would varnish their plates to protect them after drying. However, there are VERY few people who still shoot wet plate, between the amount of equipment (portable darkroom) needed and the fact that the process is so dangerous (ether, HCl). It's possible that the person could also be shooting dry plates, though I'm not entirely sure whether one would varnish a dry plate.
A totally wild and crazy idea take some extra prints and try some of the products mentioned. If they are ruined you can always reprint them. If you find a product that works then you will have the answer for the customer.