ditto what the others said. Get a new gauge FIRST.
If the person is using some heaviy oil.. like 20w50 or 40w and it's coold, and the engine is reasonably tight, then when cold at high idle you can for sure get near 80 for a bit.
It should be closer to 55 on a tight engine once she warms up.
Till that gauge is changed.. you can't rely on it.
Finally.. YES.. it could be a stuck relief.. but I'd address the other issues first.
If it was suspected it was a stuck relief, I'd drain the oil in it if it was any of the heavy stuff mentioned, then I'd go buy 4 quarts of store brand cheap 10-30 synthetic engine oil, and a pint or two of seafoam.. and dump her in and let her run at low idle ... might just disolve or soften some parafinnic solids and let her start working again.
if that worked. Drain the oil.. change filter, and if it was me.. I'd go back in with the store brand 10w30 synthetic. store brand synthetics are only a little more per quart than name brand dino oils.
The last 3 tractors I've done any valve / head work on, have gotten synthetic cheap store brand oil and have had 0 problems for the last few years.