How can I protect the mixed-media surface of my table?
-Note-: I started this answer before there was mention of potential of varnish on the table. This answer would hold outside of that. 

While you can usually epoxy over almost anything if you already have a varnished surface you might not need to bother. "Varnish" a little vague so it would be hard to be exactly sure. I would like to think the artist is aware of this so it should not be an issue. 



I think one of the better choices here would be epoxy resin. You could also call it a bar-top finish. They are essentially the same thing. I had some initial hesitation in offering this suggestion until I saw multiple artists used epoxy resin to give a glass finish to their own acrylic paintings. (One such example)

They are meant for strength and longevity. However they are more susceptible to sunlight and UV rays then other finishes. You see this in other crafts as well like penny tables. Epoxy resin is usually food safe once fully cured check your individual product information to be sure. . 

If you have never done this before this should not be your first large project with epoxy resin

You need to work in an area that would be relatively free of dust and debris. Epoxies come as two liquids that need to be mixed together: a resin and a hardener. Most require a mix of 1:1.  Hard to say exactly how much you would need to cover your entire surface.

If you can remove the legs off your table I would do that and then use some small blocks or other objects to prop the table off the ground. Epoxy resin is self leveling so it is imperative that you ensure your table is level before application. Use a level or series of level to help ensure that. At the same time you need to put down something under the table so that the resin has something to drip onto. This is supposed to happen to ensure an even finish. 

Mix/stir your epoxy resin and pour over the table. You can use a brush to help the resin get around the whole surface. You are not "painting" the surface but helping the resin get complete coverage. Again, it will drip over the edges and that is fine as it helps coat the edges. You have a few minutes so if you need to add more resin to your surface you have a small window to add more without harming it. 

Bubbles are bound to happen so have a blow torch or heat gun handy to help pop them before the resin starts to cure. I even heard breathing (not blowing) on them helps them pop. This can especially be an issue for a mixed media table as there will be plenty of places for air to hide. Make sure that whatever you are using that you don't want near surface contact and stay in motion. You don't want to burn it or cause wrinkles if the epoxy is starting to cure. 

You can sometimes thin your epoxy to give it a longer curing time which both allows it to seep better and gives you more time. Note this can affect its overall strength. 

Once you have your surface coated and bubbles removed then you typically would leave it to cure over a couple of days to be sure. If you have the means to tent the table this will help keep dust off. Be sure whatever you do that there is no risk of the blankets or what not falling onto the table.

Epoxy resins can be a complex subject if you want them to be. There is a lot of scenarios that would be too hard to cover hear. Do you research and always pay attention to product labels. So, if you are curious about this I would recommend reading up on it more. If you wanted to explore this in more detail I would recommend asking specific questions about your particular quandary. 

Art-Resin has some starting videos on the subject.