The father told Michael he should find

[Bassim]

I am wondering if my sentences are grammatically correct. Is it correct to use "once" instead of "friends" in the second part? Is "leave him at the first obstacle" correct English?

The father told Michael he should find some real friends and not fair-weather ones who would leave him at the first obstacle.

 

[J&K Tutoring]

It certainly is not correct to substitute once for ones or for friends! I assume this is a typographical error, caused by you being too reliant on spell-checking software.

I am wondering if my sentences are grammatically correct. I see only one sentence. It is grammatically correct, and the use of ones instead of friends avoids the repetition, but I think this sentence would still be better with friends. The idiom 'fair-weather friends' is alliterative, so works best unaltered, in my opinion.

Taken out of context as it is, the use of the father is a bit odd, and perhaps wrong. If you mean the father of Michael, then it would be better as: Michael's father told him...