Translation:This relationship is believed to make his mother happy.
A word of caution when using "Hubung" = "connect", I had an awkward moment once when I meant to say to a female colleague "Let's connect later" which is perfectly fine in English but in Indonesian the active transitive verb form of meng+"hubung" can commonly refer to sex. Note it can also mean connect but people don't really say "Let's connect later" in a business context.
What about "It is believed that this relationship will make his mother happy."? Otherwise, there is no future setting in the English sentence, and it is impossible to reverse-translate.
Another less graceful but closer translation: "This relationship is believed that it will make his mother happy." Very awkward, but grammatically correct. Yet not accepted.