In terms of grammar, there is nothing wrong with the two sentences; their meanings, however, are
very different. The first sentence means that Sunita no longer likes Rajender.
When you `go off someone’, you stop liking him/her. It is also possible to `go off something’ as well. The expression `go off someone/something’ is mostly used in British English in informal contexts.
Example: Ganesh started to go off the idea of building a new house.
The second sentence, `Sunita has gone off with Rajender’, suggests that Sunita has accompanied
Rajender to some place. When you `go off with someone’, you run away with the
person leaving your family behind.
Example: The rumour is that Sridhar has gone off with Geetha.