Share it = yes
For Free = no
Charge £6 a month for a throttled bandwidth of 20% of your line speed (£30pm divide by 5 = the 20% at £6 per month)
If he does not pay one month just change the Security Key or Ban his traffic.
Simples
I did this in a flat i lived in once I ended up making money on it too

block of 6 flats I lived in the middle paid £50pm for 50Mb Virgin Broadband TV & Phone.....
One guy say my Wireless and asked if he could use it I give you a tenner he said ok took the tenner and gave him my key and said it will expire at the end of the month he did this for 3 months on the trot when the other flat residents asked for the same deal.
So I setup my Awsome router (not the Virgin one) to dish out 6 SSID's
Flat 1, 2, 3, 4, 5 & 6 I throttled mine Flat 3 to have 70% of the line speed and i split the remaining 30% between the other 5 so about 15Mb between 5 so they had about 3mb each they each paid me £10 a month so the £50 i was paying they was coverd I loved this arrangement not one of them complained about speed issues at all
I made money when I got an offer off Virgin to reduce my monthly cost I got it for half price for 6 months so I made £25 per month for the 6 months so i made £150 and i got free TV, Broadband & Phone Line Happy Days
When i moved out they was all gutted me too as it was a good deal for me and they all loved the idea