Can 尔 (er3) be used instead of 你 (ni3)? I got a letter from a new penfriend in Hong Kong, written in 普通话. I think their first language is Cantonese. Mostly the letter uses simplified characters, with the occasional traditional one. It starts by using 尔, e.g.

尔最近的工作忙吗？

then shifts to using 你. So I was wondering about the differences between the two. It looks like for this meaning 尔 is more formal, and the dictionaries I looked at say it’s a formal word when used for “you”. But I’d like to have some examples; the dictionaries don’t give much detail here.
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".