Not many insurance companies cover for stress related illnesses any more, they'd need to check with their insurance company.

We haven't got ins yet need one as outlined in my post
Mainly one that will cover if other person had to cancel

I meant on application they'd have to ask if they'd be covered for stress related illness as not many companies now cover stress as standard. They may do so for an extra fee but again, it's up to the individual insurance brokers.

Have you tried looking on the internet at various policies?

Yes and the wording on stress isn't very clear
Apart from that we still need ins that will cover one of us to cancel if the other has to

You will need a policy that covers if a travelling companion is unable to travel.

You will both have to declare any pre-existing medical conditions which means your friend will have to declare the stress related illness and anything else.

If it is more than 12 months since your friend was ill then look at http://www.eta.co.uk as they will cover conditions provided there has been no change in medication in the past 12 months. However read the policy carefully to ensure it covers everything you need it to.