Bear in mind that the info in the above linked thread dates back to 2004
The primary facts remain valid (Pi or HSE marked cylinders in the UK) but it mentions the old practice of teams using scuba fills either because sites still ran co2 or as a 'grey area' workaround to filling a USA cylinder in the UK
Don't do it - you would be the culprit, and its cheaper to buy a UK paintball cylinder the to buy a fill system and a scuba cylinder
Again keep oil & lubes away from the cylinder
If the cylinder is within its final date then first look it over for anything obvious such as scratches, if he had had it stored out of use check if it had air in meantime. If its been stored empty then it could have got damp, rust and be on the way to failing retest
Check the remaining life, eg cost a cylinder as about £200 over 15 years, which is £13.33 per year
If it does not have more than 2 years to final expiry its not really worth testing
Add in postage on top of retest fees if you cannot hand it over and collect it afterwards