So they will look into other railway companies as well?
Welcome - sorry to hear you have had this problem whist studying here in the UK. It can be the case that railway ticketing in the UK is quite complicated and I can appreciate this is not helpful if you are from overseas and are not familiar with it.
Can you state what type of Railcard this involves? What is the name of the Railcard?
They will be likely to look into all the tickets purchased with the Railcard for travel with any train company. It will then potentially be necessary to show which of those tickets were for use by your friend who owned the Railcard (which is OK) and which were for other people.
I would suggest that whilst you wait for the letter you sit down with your friend and make a list of all the times the Railcard has been used to buy tickets, then use a diary to work out which of those occasions were tickets bought for someone other than the Railcard owner (the owner being the person named on the railcard). Then see if you can make a list to help proove that it was not you using those tickets. If your friend can proove that they used the Railcard on all the other times, for example, that will help. Though it is possible that they won't ask about this, but if they do you will be prepared.
You and your friend should also now read the T&Cs for the Railcard concerned on the Railcard website (there are not that many) and be sure you understand them.
As mentioned above, they will not be likely to believe that someone would think that a Railcard in the name of 'Joe Bloggs' can be used to buy discounted tickets for any of their friends and then take the Railcard with them to make it valid. I fear they will suspect this is a case of Railcard fruadulent use, not a misunderstanding of the way Railcards work. However, in the country you are from - does such a system exist? ie you can buy a Railcard and then use it for other people's tickets and lend them the Railcard? If so when you have to reply to LNER perhaps you could explain that by way of an explanation of your error. But you should expect to name the country, name the Railway operator in the country, and proove it eg by way of say a link to such a Railcard in that country. They might then perhaps give you the benefit of the doubt to some extent. For example if you a student from China say, do Chinese Railways have Railcards that work like this, if so you could explain that.
As you are an international student it would be wise for you to ask for some advice from your Student Union welfare office or your university / college International Student office. I think they will be able to help you with this too.
They may also be able to advise you regarding getting a lawyer as they may have a relationship with a reliable legal firm who give their students some free legal help. But you may not need to get a Lawyer/Solicitor yet.
At the moment I think you just have to wait for the letter, prepare for it by doing the above things, and then head back here for advice on how to respond to the letter. Hopefully you gave them a UK address where you will be able to receive post and that you can look out for this post as you will need to reply quickly when they write to you. People here can advise you on how to best reply to that letter when you get it.
In the meantime try not to worry too much about this, but it will be important for you to get some good help and advice.
Also head back here for advice when you need it.