What's a good app for making records of strength training?A few options I can think of:
1. Download two separate apps. Use one for your own data and the other for your friend's. I don't have any experience with Android apps but these are some that seem to be rated relatively high:
Liftbook for Android
StrengthCalc for Android
Strong Log for Android
2. Use Fitocracy for Android. It's not as straightforward and neat as the others I've mentioned above - it's more of a fitness social network. For your purposes you'd have to register two accounts, then log in and out as you require (which would be a real pain, but the data/graphs are pretty).
Fitocracy for Android
3. Create two spreadsheets and upload them onto Google Docs, then edit them with your phone.
Sorry, none of these are very elegant solutions. I can't find an app that allows you to key in two separate sets of data.