If you give her a voucher for two people is she going to assume that it's for the two of you, leading to an awkward moment?
Why would it cause an awkward moment? Doesn't she know her MIL well enough to just say 'you can take whoever you want'? A day spa I'd be OK with giving just the one voucher, if it were concert tickets or something like that I'd get them two (which I have done for people in the past as some people won't go on their own and if they can't find someone who'll pay for their own and go with them there's a chance the ticket could be wasted).
Edited by Tigerdog, 19 December 2012 - 03:56 PM.