Is this true?

My friends website alphatech1.com.sg sometimes comes up with a warning about it being dangerous - he asked his tech guys and this is what they said -
. Due to your site compromising with malicious codes last year, both your domain name and my server ip are been blacklisted by anti virus program, which also affect my other clients. Only if you change your domain name and host in other server will remove the alarm.

If this is true why does it only happen sometimes and what can he do to get rid of it?

This is NOT true. All sites can NOT be affected by a blacklist motion. 99% of the time the warning comes from Google, Yahoo or Bing. To remove this warning scan then server for malware, bad scripts and what not, the go to each engine and request a review of your site.

It is not so much about "true" or "false". There are a few reasons this may occur. One reason is if a website is using self signed SSL without having it signed by an authentic service like Verisign. This can essentially be a warning to the users who visit the site that the website is claiming to be secure signed but is not actually signed from a trusted source.

The second option is their browser is throwing a warning because the destination page runs code in a way which has the "potential" to be harmful to your computer (even if as is, it is not harmful). An example of such a website is one where when you visit it, it discretely tries to download a file to your computer. Another may be a website which executes a mallicious piece of javascript on page or DOM load.

I hope this offers a little more insight on the problem your friend was facing. It is possible that I misunderstood the question a bit, so if you have any other details you'd like to discuss, I'd love to contribute.