The cat who went outdoors came back with a whole new smell and that is what is causing the conflict. Someone once suggested rubbing some tuna juice on the fur of both cats and seeing if they will lick it off. This may be messy and might not work, however!
Probably the best thing you could do is re-introduce them just like you would a new cat. Put the "outdoor" cat in a small room and let the other sniff under the door for a week or so. During this time switch them occasionally so that each will have a chance to sniff around each other's territory. Rub one cat and then the other with a towel to exchange scents.
After a while you can open the door a bit and let them see a little more of each other and then have short supervised sessions in the same room. Try playing with a kitty tease and get them both involved.
For more information on introducint new pets check out:
Feline Behavior
You may be surprised that the re-introduction won't take long. As the "outdoor" smell wears off the other kitty may start to recognize his/her former playmate.
Good luck!
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