Paternity leave… fraud?

So question, in the US if you are on paternity leav, you get hired at a new job, start this job while getting paid your leave at the old job.

Once your leave is up, you ‘go back to the old job’ but take your sick time, then quit.

Is this legal and a smart thing to do, or would this be considered fraud?

I would like to add, this is not me or my husband but someone at my husbands company who screwed everyone there. Obviously it’s not our business, just really annoying when a company gives paternity leave to try to gain employment and look good to new employees but then gets taken advantage of.