Is it okay to send flowers to a Jehovah's Witness' memorial service?

Yes.

The funeral of Jehovah's Witnesses is primarily to provide comfort to grieving ones, and certainly flowers are something that provides cheer to those receiving them. In fact, if you go to the funeral service in the Kingdom Hall, likely you will see flowers on the platform that have been sent by others, including members of the congregation of Jehovah's Witnesses.

Edited to add that while Jehovah's Witnesses are Christians, we do not use the cross symbol in our worship. So if you do send flowers to a memorial service/funeral for one of Jehovah's Witnesses, please do not send one that is shaped like a cross. We would certainly appreciate the sentiment, but would not display such an arrangement.