Are you in England or Scotland? The rules are different in each country. The lambs need onlt slaughter tags when you sell them. These must be EID in scotland. If your friend decides to keep the lambs after july 2014 (no matter what sex or if they are castrated) they must be double tagged, one EID and one matching normal.
Importantly. If you use a single slaughter tag he can use any colour of his own double tags but if you use your double tags and the sheep loses one of them he must replace both tags with RED tags with his flock number on them!!!!