Yes you can change out that seal, its an easy job but more involved than you would think.
1. drain the oil.
2. take your clutch cover off
3. remove your foot shift lever
4. push your shift shaft through the clutch cover side.
5. pry out that little seal with preferably a seal puller, but if you dont have one a regular screw driver will do.
But make sure to place a rag folded up over the case before you pry against the case
6. install the new seal using the old seal as a driver to drive in the new seal with a hammer.
Do you know what seal he is talking about? the one you say he forgot? this could possibly be a problem but the shift shaft seal should stop your leak. If he forgot one internally, then thats a different issue that would not affect the leak your speaking of.