If you and your friend are grateful for the assist that you received, you could say; "NAME and I would like to that you for your support in BLANK". It is very formal sounding and can convey a lot of respect and gratitude. Be sure to go into detail on the assist. For example (using my friend Shigoto who helped me research for my book): "Shigoto, I would like to thank you for all of the support that you have given to me in my book. Without your fine research skills and your craving for knowledge, my book would have never been possible. There was no way for me to research in 4 months what you did in 4 weeks. Thank you so much." (be sure to speak in a formal fashion.) If only your friend is the only one expressing thanks, the pattern goes like this; "NAME would like to thank you for BLANK." Using a similar set up from above, counting speaking formally, this should be just as easy. I hope this helps.