Im not sure how these two are being kept together, as a tortoise is a terrestrial animal, and the sliders are primarily aquatic, so someone's needs are not being met somewhere.
I would first tell my friend to separate the two and get them in their appropriate captive habitats. Their health will seriously do nothing but benefit from a proper environment. That includes, temps, humidity, diet, and UVA/UVB.
I'll let one of the turtle/tort pros answer your egg question, but definitely tell her its in her animal's BEST interest to get them set up properly!
