Can frogs and turtles live together?

Yes, in fact this would be a better cohabitation than many as they would rarely, if ever, be competing for the same food and will likely be tolerant, if indifferent, to each other's company.

Do plenty of research on mutually suitable substrates and terrain (avoid too many items that would inhibit the tortoise's movement...low rocks might be good) and give them plenty of personal space, hopefully in a 20 gallon or larger tank. Obviously length and width of tank is more important here than height. Also, I would recommend loose "woodland soil" as the substrate so they might each dig and bury at will.

Also, note that the snake will probably be oblivious to the tortoise's food and will likely knock it into the substrate as it moves around, so look for that periodically to avoid rotting vegetation.