I know coupons must be used in order, but what happens if it is with different airlines and they have different PNRs . Basically there are 2 sectors my friend don't want to use:

Currently the situation is that he has a ticket:

a-b-c-d-e

he wants to get from b to d without going through c and he will pay for an oneway fare for a direct flight b-d.

bc and cd are on 1 airline and d-e is a different airline.

If he turns up at d to get the d-e flight, do you think there will be any problem. If worst comes to worst he can do the re-routing at the airport?