I've never seen a Specified Illness Cover plan from any insurance company that didn't include stroke as one of the illnesses, although you'll need to see the policy conditions. While there may be conditions relating to severity, from what you're saying I suspect that this will be a valid claim.
When she started the policy, she should have received the policy documentation, including the policy conditions booklet. If you can't find that then you'll have to ask Ark Life to send you a copy, along with a claim form.
Hope your wife is doing okay.