How can one return the kindness to someone beloved in God, contribute to their entry into Paradise, and what are the ways to benefit them in this world and the hereafter, other than supplicating for them?

The summary of the answer is:

A Muslim should love his Muslim brother with a sincere love for the sake of Allah and inform him of this love. One of the greatest aspirations is to be a cause for someone entering Paradise, for in that lies true happiness.

Among the most important ways to help your brother is to supplicate for him in his absence, for the supplication of a Muslim for his brother in his absence is answered.

You can also help him to be steadfast and cooperate with him in righteousness and piety, for the one who points to good is like the one who does it, and whoever guides someone to good will have a reward similar to his.

If you both love each other and meet for the sake of good and part for the sake of it, you will have a great status on the Day of Resurrection, as mentioned in the hadith about the seven whom Allah will shade with His shade, among them "two men who loved each other for the sake of Allah; they met for His sake and parted for His sake."