回复：H1延期间离美，新加坡签H1被拒绝，F1入境美国，

Probably his H1 extension would be denied as the base of H1 extension is lost (no H1b anymore after he uses F1).
Somehow he may still receive H1 approval notice. Then he can leave US and get a stamp for H1 re-instatement.

What was the reason for H1 rejection in Sigapore? If at the time his old H1 still valid, should be able to get in.