Thats my thinking.
In in £s or $s ?
Well I imagine a personal cheque would need to be in £sterling. I know in the UK it is possible to bank a foreign currency cheque in to one's own £sterling account. I've no idea if it's possible in the States.
If money transfer via Western Union is not possible the other suggestion I have is a $ travelers cheque but this would probably have to be sent signed by the purchaser in the mail. Even then a US bank might look twice at it.
If the lady in question wants to do this herself how about this. Get her to give you the £100. You get the banking details etc. to use online banking and send from your own account BUT let the lady in question press the send button. As an aside we send £50 every year to a nephew in South Africa via online banking, it really is the only solution these days.