If the non-hike user is a DND (do not disturb) registered, all that s/he needs to do is to opt-in to hike before you both can start communicating with each other. By opting in to hike, your friend is not opting out of DND, but merely allowing hike to send and receive messages from you.
How to opt-in to hike?
A non-hike user can opt in to hike by giving a missed call to +919266630847 from his mobile. Dialling to this number, will automatically, opt in the DND user to hike and you can send him Hike2SMS messages.