I often think of those good old days...,

[Silverobama]

Mike told me last night his English club would be suspended because the current venue of the club was going to close. As a friend of him, I hadn’t been to his club very often to support him because I was too busy. I apologized for that and promised I would go to his new club once I’m available.

I also said to him “I often think of those good old days when we chatted with each other in English at Koala coffee shop*; the memories were mesmerizing”.

Is my italic sentence natural?

*：Koala coffee shop is the place where Mike and I first met and we often went; an English-only place.

 

[tedmc]

Yes, but I would write "as his friend" instead of "as a friend of him".

 

[emsr2d2]

Mike told me last night that the venue where he runs his English club would be suspended because the current venue of the club was is going to close, so he has to find a new venue. As a friend of him Despite my being a good friend of his, I hadn’t been to his club very often to support him because I was too busy. I apologized for that and promised I would go to his new club once whenever I’m available.

See above.

I also said to him “I often think of those good old days when we chatted with each other in English at Koala"; coffee shop*; the such lovely memories!" were mesmerizing”.

See above. BrE would generally use "chatted to" but yours is also correct. If both you and Mike know what Koala is, there's no need to add "coffee shop".

Is my italic sentence natural?

See above. "The memories were mesmerising" really doesn't work!

*：Koala coffee shop is the place where Mike and I first met and we often went; it's an English-language-only place.

 

[tedmc]

I think "mesmerizing (captivating)" as an adjective for "memories" is a bit over the top but I wouldn't go so far to say it's wrong.

 

[jutfrank]

To describe memories as 'mesmerising' is highly inaccurate.

 

[tedmc]

To describe memories as 'mesmerising' is highly inaccurate.

I agree it's a highly unlikely collocation but it has been used:

https://yaktack.com/words/mesmerizing%20memory

Logically, you can only be mermerized by something that physically happens in front of you, not by images in the mind.

I think "vivid/cherished/warm/pleasant memories" would be more natural.

 

[güey]

but it has been used

In Indian English (or other South Asian), perhaps -- that's the sources where it comes from.