You can charge by the hour if it's only 1 house you are going to or figure out a half-day/full day rate for more than 1. You should also try to do a recce of the client's house to check lighting, compositions, etc. Just 30-45mins should be enough, good if you can go with your friend so he can direct you what's important to photograph. Take snapshots, notes, go home and plan. You may need a stylist/or your friend on the actual day if you don't want to try styling on your own.
Actual shoot hour(s) + recce time/planning + post process + transport/prints/hiring extra equipment/hiring stylist with surcharge. Provide an invoice at the end.
That's the whole routine, if you're afraid of charging your friend too much then adjust your shooting rate accordingly, but at least recoup on money spent. You're doing this for his portfolio, and could become your portfolio as well.