To properly understand it, you need to differentiate between BITS and BYTES, where one BYTE is equal to eight BITS.

The initial speeds you mentioned (your 512Kbps and his 10Mbps) are measures in bits - 512 Kilobits per second, and 10 Megabits per second.

Now, to work this into download speeds, you divide those numbers by 8, to convert into Kilobytes/Megabytes per second.

So, your maximum theoretical download-speed will be 64 Kilobytes per second, his will be 1.2 Megabytes per second (or about 1230 Kilobytes per second).