Can 尔 (er3) be used instead of 你 (ni3)?
尔 in traditional chinese is written in 爾. Although it also mean "you", I believe it's a typo. Because 


尔 is widely used in old chinese. The situation is just like "Thou" and "you".
It's rarely use in nowadays daily life no matter in HK, mainland China nor Taiwan. 
Instead of simplified Chinese, Hong Kong people usually write in traditional chinese. Sometimes people tried to write in simplified chinese quite some HK people would mix up some wordings. I would rather believed that is a typo if you said your pen friend is come from HK.


When will we use 尔?
One common example we will definitely use nowadays is 尔虞我诈(or in traditionally chinese is 爾虞我詐). Which is an idiom mean "People cheating and getting advantages of each others".