I probably just don't understand the rules around suffixes yet. But I would have assumed "his friend" was صَديقَهُ not صَديقَتهُ. Could someone explain why the ت winds up in there?
Samia is a woman, so صَديقَتهُ "his (female) friend". صَديقَهُ is "his (male) friend".
Let's put it this way: صَديق = (male) friend and صَديقَة = (female) friend; adding a suffix leads to صَديقهُ = his (male) friend and صَديقَتهُ = his (female) friend. Since Samia is a woman, you have to use the feminine form of friend, thus صَديقَتهُ = his (female) friend.