As-salāmu ʿalaykum (Arabic: ٱلسَّلَامُ عَلَيْكُمْ ) is a greeting in Arabic that means "Peace be upon you". The salaam is a religious salutation among Muslims when greeting, though it is also used by Arabic speakers of other religions, such as Arab Christians. The typical response to the greeting is wa ʿalaykumu s-salām (وَعَلَيْكُمُ ٱلسَّلَام ) "And peace be upon you, too."
Yes. Responding to the Salam is obligatory, although offering the Salam is recommended but it is not recommended for men to offer Salam to non Mahram females.
It is obligatory to respond to the Salam as it was given, and it is recommended to add on it to make it a better response. If you were been greeted by (Salam), you must respond with (Salam) as minimum response, but adding Alaykum on Salam is better.
Simply reply by Alaikum Assalaam wa Rahmatullaahi wa Barakaatuh
in fact this is a dua
you are saying "May God's peace and blessings be upon you" in Arabic
So if this friend of yours is acting that much friendly and wants to greet you with your Islamic culture do treat him/her in the same way and DO translate it for him/her.
as salam alaikum
the Qur'an says: "And whenever you are greeted with a greeting, greet with what is better than it or return the same" (4:86).
Therefore in the above-mentioned case a full response should be given although it is preferable even a better response like "wa alaikum salam wa rahmatullah" or "wa alaikum salam wa rahmatullah wa barakatuhu".
With prayers for your success.
Thank you for your question. It is generally praiseworthy to be an approachable and kind person in normal circumstances. However, that is within the bounds of wisdom and chastity. It is not necessary to reply a typed salaam whether that be by email or other forms of instant messenger.
May you always be successful.