hi buddy
get him to take it to a gun shop to get re-filled,and make sure he cocks it first! other wise it will not fill
then go round and have a go on it at least 3 mags,if ya happy with it and no leaking air,then buy it.
the rust is easy to re-move if its slight rust,if its pitted on the cylinder/barrel then my advise would be most importantly to change the cylinder! the barrel can be sorted
PS if ya need to change the cylinder,contact me first buddy
or search davyt63 on youtube,you will see the footage i have done on cylinder change!
regards
davy