Well speaking as an instructor who has sent a few first solos the PLAN is for him/her to fly ONE circuit and landing only - any more than that is tempting fate methinks. The second solo is a MAX of 3 circuits - I stress to the student that if he has had enough after one thats fine. Next details he can taxi back to the holding point after MAX 3 circuits for some more - throughout all this I am watching him carefully from the ground - ideally in the TWR.
When you send a first solo, in particular, you pay very careful attention to the weather, remaining light, traffic in the circuit etc. - most of the decision making as to whether he/she ought to go is made by yours trully!
Of course, there is always the "Murphy factor" of something going wrong but a good instructor can eliminate a lot of these factors by being careful. If there is any doubt don't send them.
It goes without saying that the student should stand a fair chance of dealing with emergencies such as EFATO but I think it's really a case of "risk management" - there are occasions with certain students where it's now or never and, as my dad who was a veteran light aircraft instructor used to tell me, we tend to forget the sheer self survival aspect when the student is on his own!!