You need to give credit where credit is due, if you are presenting someone else's work and they put in the majority of the work you must share that. Yes you may have been part of it and perhaps even reviewed it, but you must give them significant credit.

Nobody likes people who take credit for others work and it will be quickly found out. Particularly if the work gets critiqued and you are asked to stand by it.

This isn't some fancy law, but general decency.