The only device used to access instawallet was an iphone that he had control over at all times between first accessing the account and the theft, so there is only two possibilities here:
- There is an iphone wallet stealer trojan or similar in the wild.
- Instawallet was hacked or the theft was internal.
My advice is to not use this kind of service, use something where only you can control the private keys to eliminate one risk factor.
If you need to use instawallet or similar services, send the coin somewhere more secure as fast as possible.
There is another possibility: was he possibly using a wifi network? It is incredibly easy to hack routers using WEP/WPA/most wifi encryption algorithms to intercept information. In fact this is one of the most widely used, and easiest, ways to steal personal info.
Please people never use WEP(cracked in 1997, considered exceptionally compromised, yet is still widely used).
Try to use WPA2 or a direct connection while sending passwords over any network. If you aren't sure; do not use it! It is worth the annoyance. You do not want to risk using a wifi network that is not properly encrypted.
Stay safe, and keep updated on the latest security issues
Your friendly online battmann.