Did some testing yesterday with iphone 5s running ios 9.2.1 and discovered a temporary fix to the 'not receiving messages from iphone users' -issue. Here's what I did:

Create a group between iphone user and whatsup user using iphone to do this. After receiving a message in this group, you are able to receive private messages as well and you can delete this group afterwards. So far this seems to work.

So maybe send a text to your iphone friend asking him / her to create a group that has you and your friend in it. If this works, you can delete this group afterwards.

Good luck!