The derailleur does not lead the chain onto the cogs when pedaling in reverse, it is on the wrong side of the cogs. Do not adjust it, The spring tension in the derailleur is backwards and will not keep tension on the chain if you pedal backwards fast. Do not adjust the derailleur. (If it really is OK when going forward)
However they may be something wrong, there may be a stiff link, that should be checked. But it will do that when pedaling forward too, but maybe it's hard to notice.

If he's cross chaining or the bike is not fully shifted or there is a sticking cable that could be it too. It could be lots of things. That means it is not quite perfect when pedaling forward. Or maybe the freewheel is sticking, that happens with cheap bikes. Then you pedal it forward in the stand and let go of the pedals. To see if they spin hard. I repeate the deraileur does not guide the chain onto any sprockets in reverse.

The key may be that he says it happens in the same part of the chain each time. Maybe there is a slightly sticky pin. The above post is correct too.