No matter how hard he tried, he couldn't help ______ his friend's success.

Can't/Couldn't help (phrase): 
Meaning - If you can't/couldn't help something, such as acting in a particular way or making a particular remark, you are/were not able to control or stop it. 
- এ দুইটি phrase এর পর verb + ing হয়।
Example: He cannot help telling a lie.

Complete Sentence - No matter how hard he tried, he couldn't help envying his friend's success.

• তবে, Can't but/ Couldn't but এরপর verb এর base form বসে।
Example: She could not but congratulate him.

Source: Cambridge Dictionary.