When Rong Mo arrived in the capital, she found out that Granny Shang wasn't sick at all.
She was feigning sick as an excuse to trick Shang Mo home for a matchmaking session. There was a grandly dressed woman with a purple dress that hugged her bosom. Her snow-white skin was covered with a faint makeup as her long hair rested gently against her back, looking beautiful and mesmerizing without the need for any accessories.
Rong Mo did not know whether Shang Mo had guessed this from the start and hence the reason why he said that granny wasn't severely sick and merely needed someone to accompany her.
Since granny tried to trick him home, he tricked Rong Mo home instead.
As for the issue of granny wanting Shang Mo to get married while he himself didn't want to, that was something that no outsider could help with.
Looking at Granny Shang trying to make repeated calls furiously, Rong Mo merely felt helpless.