Well, it doesn't sound like a good idea. I doubt that you can find many legit wallets on Play Store for your phone.
Offcourse, you can download .apk files of old versions of some wallets, something like @hugeblack posted above. But I wouldn't risk to use outdated version of wallet.
IIRC, schildbach wallet was the first android wallet available... I have no idear what the minimum android version for the current release is tough... No harm in trying it out (i guess). AFAIK, it doesn't have any vulnerability's (but, truth be told, i haven't used this wallet in ages, so you should DYOR before testing)/
Oldest Android version supported by this wallet is 6.0
Rather than looking for wallet which can support old version of android, you should look for custom ROM (which contain newer version on Android) for your smartphone.
Android 4.3 is very old version and probably best what he can do - update it to 5.0, not more. But it wouldn't help him, because 5.0 isn't supported by most wallets too.
He can use this phone as airgapped device, but personally I wouldn't trust such old phone for it.