Androids are full of malware and who knows if his seed was leaked in some way. Even deleting the wallet might not remove all traces. The hacker might just wait for a deposit and then sweep all the funds.
He could flash his phone, insert an OTG with the wallet apps ready to be installed. Install the apps, generate new seeds, store it, and leave it. Uninstall the apps, reflash the phone and done. But this is inefficient and ineffective. Agree with the HW part, if the point is having a cold wallet, a phone is not the best choice.
the real question is why bother with this much workarounds just to increase the security of something that is inherently insecure by design. in other words unless OP doesn't own a PC or has no kind of access to one whatsoever, i don't see any reason to take the risks of creating a wallet with a phone for storage purposes.
HW wallets would also require a PC so if he has one then he should use that to create a cold storage.