"P.s. everyone saying they should have split the vouchers- you do realise you have to pay for the vouchers? confused why should they split vouchers they've paid for?"
They didn't pay for them.
They collected them for free from Tesco and then exchanged at 4-to-1.
They aren't the same as cash.
secure.tesco.com/clubcard/boost/eating-out/4294967291.cat
Tesco don't pay the restaurant full price for the vouchers, because the places you can use them at are chain places where there are loads of discounts anyway. So the '£10 voucher for £2.50 of voucher' is misleading.
It's very different from, say, a Gordon Ramsay voucher - I bought one for my parents as a gift, and you pay the full price of the meal with no discount.
Basically the OP's friend has got some vouchers they want to got rid of, and invited the OP along to cover their service charge and drinks, these vouchers are ONLY valid for food, and they DON'T cover the service.
So basically what's happened is:
OP's friend has bought £40 of vouchers to shit restaurant for £10, but they don't cover the drinks or tips.
So OP's friend has invited OP along in order to exchange their shitty Tesco voucher for cash.
Which is not on.
Obviously the Tesco vouchers have value, but it's not £40, and it's not on to invite people to a shitty restaurant so you can use up your vouchers.
They are cheap spongeing bastards.
At the very least if you want to use your shitty Tesco vouchers offer them at a reasonable price, like 'I've got £20 of vouchers, do you want them for £15?'
Also if you are deliberately spending more to use up your shitty Tesco vouchers, then it's not on to then split the bill with people who are paying with actual cash.
It's a different dynamic from a group where all are paying cash.
Fair IMO would be offer £20 of vouchers to the OP for a discount, somewhere between £10 and £15, and then OP pays her share, so
£15 to OP for £20 voucher
plus £11.57 for the remainder in cash
that's £26.57.
At the most.