It is inevitable, every year the cost of living rises. One of them is marriage costs. Couples who are going to get married should be able to set their budget so that they will not be burdened after the wedding. Yes, they should honeymoon, not think about the reception debt.
It’s sometimes difficult to invite a friend and ask them not to bring a guest (that is to send an invitation to your friend and not to ‘your friend and a guest’) because one usually thinks that an invitation, by default, is for you and a guest (a date, if you will). The problem with that is if everyone you invited brought a guest, the cost of holding the wedding would increase tremendously.
To be entirely honest, I think every guest would understand if you merely extended an invitation to him or her and not to an accompanying guest (we can all understand how much a wedding costs). Now, if your friend is married or has a serious boyfriend or girlfriend that you know, if might be difficult to pull this off and its recommend that you invite them both. It’s not worth the grief of inviting one and not the other. However, if the financials really are that tight, then just invite one as they will likely understand (if they are your friend, which they should be if you’re going to invite them!).
Simply send an invitation for your friend and then follow it up with an email after you’re certain they’ve received it. Explain that you are looking to keep the costs of the wedding manageable and that you mean no disrespect but you only invited the friend and not a guest. If you are good enough friends with them to invite them to your wedding, they will likely agree. Don’t be afraid to broach the subject!