If he will be able to join us

[Xinfu]

-It will be splendid if she will join him.

Is my usage of the second WILL correct? If not, why did my teacher tell me we can say this?:

-He will come if it will be any use to her.

[Rusty]

In the first sentence, the use of the second 'will' is required.
The second sentence is correct usage. Most folks place 'of' in front of 'any' when using American English, but either way is understood.

[Xinfu]

Thank you, Rusty~