Checking in baggage during a Transit

Hi

A friend of mine is flying from Newark to Lagos via Heathrow. He is going to be in transit for about 9 hours. I would like to send a suitcase to an acquaintance in Lagos through him whilst he is in transit. Can he check his luggage in all the way from Newark to Lagos and then check my suitcase in during transit at Heathrow?

He has got a UK visa so he could come through immigration to pick the case up from me and then check it in as normal himself but not sure if this is allowed given that he has already checked in luggage in Newark.

Thanks in advance for your help!!

1. depends on his baggage allowance but yes it is allowed

2. and his honestly being able to answer the security questions about baggage. 'Has anyone asked you to carry for you?', 'did you pack the bag yourself' etc etc