Many people experience bloating
after quitting smoking
, which may last for up to few months. Ask her to take some antacids and to include fibrous foods in her diet. Ask her to do light exercise like walking, or any other exercise which her cardiologist has allowed. Weight gain is also common after quitting. The pain may be due to gaseous distension of the stomach and if so, will improve once constipation is relieved. If it doesn't improve, please consult a doctor as it may be something unrelated to quitting smoking.
Tell your mother that these symptoms are temporary and will eventually improve, but the benefits of quitting smoking are immense and certainly worth all this trouble.