A friend had Bitcoin stored on an old iPhone (back from 2012), in an app called «Bitwallet» (by Sollico software).
But when they tried to transfer it out, it complained about the key being "neither a compressed or uncompressed key".
[...]
We also have the public key/address. So what we have (if I get this right) is the public address, half the private key, and the checksum.
Any reasonable way to get to the coins with this? Any other ideas of what to do? There's 3 BTC on there.
It all sounds a little suspect if you ask me, and the concerns have already been expressed. I don't think this is a "friend" of yours and you are concerned about his welfare. Rather, it gives your impression that you are only interested in the balance of this wallet. If it is
this address 1FYMZEHnszCHKTBdFZ2DLrUuk3dGwYKQxh, then the question is why your friend is running a 10+ year old wallet on a 10+ year old iphone and is now suddenly interested in withdrawing the funds and even though coins are coming into this wallet on a regular basis (most recently this month).
I think there is nothing more to add here.