Answer: No. He was not.
Step-by-step explanation:
Roy, in this case as a third party was simply a TENANT - IN - COMMON with Stella and is not entitled to the RIGHTS OF SURVIVORSHIP.
In a Joint Tenancy, there is a common ownership of property with the Rights of Survival. If Party A and B have Joint ownership and Party B dies, Party A automatically takes over the property. That is the Right of Survival.
However, third parties do not have such rights because they did not take the title at the same time or with the same instrument. When sold an interest they simply become a Tenant in Common with the remaining owner.
So even though Ruth sold her interest to Roy, upon Stella's death, the property passes in it's entirety to Ruth as Roy was just a Tenant in Common, not a joint owner.
If you need any clarification do react or comment.