I'm a bit wary here, being a believer that nothing in life is for free!
I'm also wary of living with close friends. Of course it depends on what your personalities are, but can see loads of pitfalls (for me anyway) and I wouldn't risk my close friendships, because if they go sour you can rarely get them back.
I've no experience of farmimg but I know it is hard work and long hours, and sometimes all hands on deck.
What will you have to do? Looking after their DD? How often? That in itself might become a full time job that you have to accomodate around your own.
What kind of building it this? Are we talking about a big ramshackle farmhouse - that will cost an arm leg and both kidneys to keep even barely warm? You need to find out what the running costs are, and who is responsible for the repairs and upkeep.
It may well be that it is for the tenant to sort out and that may in turn be you.
Finally I would think about what you are giving up. Yes you would be able to leave and go back into private rental, although it might spoil your frienship, but do you have a good LL/house at the moment that you would be giving up?
Some LLs are ace, but as you see on here time and time again there are still a hell of a lot of bad LLs doing the rounds. If you are happy where you are now and with the LL, you would risk putting yourself back at the mercy of private rental and trying to find somewhere decent to move on to.
And as has already been mentioned, do check the LL/owner is aware.