Cheers dude. So I only need one Core client and just ask it to boot a different wallet each time?
Correct, or even better after readin OmegaStarScream's post:
The last version (0.15.0.1) support multi-wallet apparently (more will come in a future version):
Multi-wallet support
--------------------
Bitcoin Core now supports loading multiple, separate wallets (See [PR 8694](https://github.com/bitcoin/bitcoin/pull/8694), [PR 10849](https://github.com/bitcoin/bitcoin/pull/10849)). The wallets are completely separated, with individual balances, keys and received transactions.
Multi-wallet is enabled by using more than one `-wallet` argument when starting Bitcoin, either on the command line or in the Bitcoin config file.
**In Bitcoin-Qt, only the first wallet will be displayed and accessible for creating and signing transactions.** GUI selectable multiple wallets will be supported in a future version. However, even in 0.15 other loaded wallets will remain synchronized to the node's current tip in the background. This can be useful if running a pruned node, since loading a wallet where the most recent sync is beyond the pruned height results in having to download and revalidate the whole blockchain. Continuing to synchronize all wallets in the background avoids this problem.
That's assuming you are both using Bitcoin Core. If you are using different wallets then you shouldn't face any problems as each wallet software put the files in a different location in AppData.
Didn't read the release notes yet, so, OP, even with an old version you could load the 2 wallets, but you could only load one wallet at a time... If you upgrade to 0.15.0.1, you can even load the 2 wallets at the same time (which might be confusing since you'll be managing both your own, and your friends funds at the same time in this case... So it might still be a good idear to load one at a time)