Can you Help me be respectful please

[KellySeveride]

A devout Muslim man I know has just achieved something very great, I would like to leave a message of congratulations followed by may Allah be with you. However I absolutely do not want to be disrespectful, so I wanted to check I will not offend by doing this. And if not what is the best thing for me to say?

TIA

Just congratulate him on his achievement.

Why the need to bring Allah into it unless the achievement relates to faith in some way?

Do you have a religion? I wouldn't be offended but I’d find it a bit strange. I would see the good gesture behind it. Why not just write god? Or a congratulatory message? How do you know this person?

Not offensive at all! That's very thoufhtful of you.
Most of the time we say Mabrook meaning Congratulations. In Arabic.
Most devout Muslims say "Allahumma Bareek Lahou/Laha Amen. Allah bless you (male/female) Amen.