A couple ways you can do this.
If using LW 5, see
Friends - LimeWire
Whether using LW 5 or LW 4, you can swap Magnet Links. ie: you create a magnet link & send it to him. He can then enter this Magnet Link in his browser & LW will open & attempt to download the file if both of you are online at same time. He can then browse you. And if you notice he is downloading from you in your upload window, you can attempt to browse him. Of course he could send you a magnet link also, vice versa.
How to use Bitzi Web Lookup ... & How to use Magnet Links & Direct Connect
If using LW 4, you can attempt Direct Connect with your friend. This would be easiest if he has a static ip address. He must be online at that time.