If it's before he's checked in, you can call the hotel and say "Hi, I'm just double checking my colleague is booked in for the night - it's Harry Smith and the reference number is 1234567."
If he's already there, call the hotel and ask to be put through to his room. Note: not all hotels have phones in rooms these days. Travelodge, Premier Inns, Ibis Styles, Days Inn (basically the budget chains.)